What are Alluvial Soils and why are they Important?

Alluvial Soils

Alluvial soils are soils transported and deposited during flood events. As such, volume and frequency of deposition varies per event, often resulting in 'stratification' of soils, or distinct layers, visible when looking at a 'soil profile' - or horizontal cross section of soil layers, distinguishable by color differences among the different layers. As time passes, various weather events - the drying out of soils by the sun/wind, freezing and thawing cycles, breaks down larger soil particles, creating opportunities for future flooding events to deposit various nutrients from broken down larger soils further downstream. Alluvial soils in floodplains are nutrient rich soils, resulting in fertile areas highly suitable for crop growth. Often in developed lands, soil deposition occurs at a faster rate and can do more harm than good when this soil deposition is a result of massive amounts of erosion or transports high nutrient loads from agricultural lands (fertilizers and nitrogen) further downstream.

Within the Fourmile Creek Watershed, two main soil regions exist: Loamy Wisconsin Glacial Till and Loess Ridges and Sideslopes. The images below are from USDA NRCS document discussing the various soil profiles in Iowa. The top two images describe the main two soil regions within the Fourmile Creek Watershed.

(Figure 1: Description of Soil Regions)

(Figure 2: Map of Iowa Soil Regions)

Alluvial Soils within Fourmile Creek

Iowa DNR has established an online mapping program to determine if alluvial soils and/or alluvial aquifers exist within an area. They have a very specific reason for making this information available online: Proper Animal Feeding Operation (AFO) Siting Atlas. Iowa State Law prevents livestock and poultry producers wishing to construct or modify an existing feeding confinement area, open feedlots, or a manure storage structure within a 100-year flood plain. (a 100 year flood plain simply means that each year, there is a 1% chance - 1/100 of a flood event occurring within an area; as such a 500 year flood plain means there is a 0.2 % chance of a flood event occurring with an area). The law is intended to prevent contamination of a water source. 

I used their Siting Atlas to clip manually clip my own areas of interest - the Fourmile Creek Watershed (Figure 3). As depicted, land within/adjacent to the stream all contains land within alluvial aquifers/alluvial soils. Additionally, Iowa DNR provides siting information with information including distance to major river, distance to surface water, percent slope, and well distance (Figure 5). In order to obtain this information, you have to select an individual point on the map - this is where the useful ness for agricultural operations comes in. They are able to use this Siting Atlas to find a specific point on their land to preliminarily determine if the lands lie within the alluvial soils area. Figure 4 shows a zoomed in version of a subsection of the Fourmile Creek Watershed, highlighting to areas delineated as having alluvial soils and alluvial aquifer. I chose a section near agricultural lands given the intended utility of this Siting Atlas. 

(Figure 3: Map of alluvial aquifers and soils within the Fourmile Creek Watershed)

(Figure 4: Zoomed-in map of a subsection of the Fourmile Creek Watershed)

 

(Figure 5: Point-specific siting information)

Sources:

Iowa DNR: https://www.iowadnr.gov/Environmental-Protection/Land-Quality/Animal-Feeding-Operations/Mapping/Proper-AFO-Siting

Iowa DNR AFO Siting Atlas: https://programs.iowadnr.gov/maps//afo/

Iowa Soil Associations: https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_006362.pdf

USDA NRCS: https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_006038.pdf

https://soilsmatter.wordpress.com/2020/02/15/what-are-alluvial-soils/

https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/alluvial-soils

Flood Plain Mapping

Flood Plain maps:

Flood Plain mapping is important for various reasons. For agriculture, it's important to know what areas may inundate from time to time, possibly reducing crop yield. For home owners, it's important to know not only what your risks are, but some localities require you to have flood insurance as a result, and the premiums for this can be high. It is important for understanding water flow, especially when it comes to nutrient loads, sewage runoff - mostly older systems - and other sources of pollution into waterways. Des Moines established a flood-program buyout plan back in 2018 following a major flood event to purchase land back from homeowners at 110% of the assessed property value. There are also projects in place to both reduce the amount of nutrient run-off into the watershed. 

The city of Ankeny, one of the areas in which the Fourmile Creek flows, established the "Ankeny Plan 2040" in 2018, highlighting a plan for various aspects of the growth of the town including topics such as the Environment, Population & Growth, Transportation, and Infrastructure, among 10 other chapters regarding the future of the city. Chapter 4: Environment covers extensively future plans for essentially all things watershed, including vegetation management (buffers, native species, riparian zones), and storm water management, providing an excellent summary of watershed management in Ankeny.

For an even MORE in depth study of Fourmile Creek, Snyder & Associates, Inc. conducted a Fourmile Creek Watershed Study in 2013, assessing numerous aspects of the stream, stormwater management properties, and land management.

Below, I have utilized Iowa's Flood Hazard Maps on their website - 'Iowa Flood Information System' to take screenshots of the flood plain. For more information on the system, see an earlier blog post I wrote on the topic. 

(Image Sources: Screen shots taken from Iowa Flood Center Mapping Online via https://ifis.iowafloodcenter.org/ifis/newmaps/hazard/)

I zoomed in specifically on a section in the northern part of the watershed with housing nearby highlighting the use of flood plain mapping in development. I've also enlarged the Legend to highlight the various delineations. The top image is a portion along Fourmile Creek surrounded by residential neighborhoods. The bottom image shows flood plain delineations. 

Sources:

2018. Des Moines Register. Polk County board votes to contribute up to $1 million for flood buyouts. Available from https://www.desmoinesregister.com/story/news/2018/08/07/des-moines-flood-buyout-program-1-million-polk-county/925003002/

Iowa Flood Center. Available from https://ifis.iowafloodcenter.org/ifis/maps/hazard/.

2018. Ankeny Plan 2040, Chapter 4: Environment. Available from https://www.ankenyiowa.gov/home/showdocument?id=5724.

Invasive Species

Invasive Plant Species in the Area

I have created a slideshow with an overview of some common invasive plant species in Iowa. I've included the ones that are commonly treated by Polk County Conservation for eradication n the Fourmile Creek Watershed area.

Prior to the slideshow, I wanted to share a video of Blank Park Zoo, located in southern Des Moines, not too far outside of Fourmile Creek Watershed. They've taken a very unique and interesting approach on tackling invasives.  They dual purpose invasive plants by removing them and using them as browse and/or enrichment.

Source:http://www.iowapbs.org/iowalandandsky/story/35326/invasive-species-removal-benefits-park-and-zoo-animals

This is an excellent idea! In this case, professionals who know what they are doing are removing appropriate tree species and being careful not to spread seed of any non-native plant. This is an extremely important note to make, as taking care not to unintentionally spread seed is of utmost importance.

Here is the slideshow of invasive species that I have put together. Below the slideshow are a few more recommendations for best management practices as well as additional resources on other invasive species throughout the state of Iowa. 

Iowa Environmental Focus suggests these steps (these are great practices anywhere)

Do not buy/sell firewood outside of your county. Emerald Ash Borer can spread this way.

Remove seeds from shoes/clothes after being outdoors as this can be an unknowing way of spreading invasives.

Remove invasive plants where you recognize them. Iowa Environmental Focus suggests getting involved with parks/groups who focus on invasive species removal. I would highly caution removing invasive species yourself  unless you are 100% of of species idea as some invasive plant species look very similar to others. Twice within the last year I have had to be careful with what I was removing to be sure I wasn't removing native plant species. I've been doing a fair bit to remove tree of heaven seedlings from an area where I currently work in Virginia and caught myself (after I pulled it unfortunately) removing a Shining Sumac (Rhus copallinum). I was more careful with every subsequent removal. The second time I was making strides to remove Persian Silk Tree (Albizia julibrissin) from an area and found many seedlings. I also noticed to that Partridge Pea (Chamaecrista fasiculata) was present in the area, discernable in my case by the tiny plants flowering and some having seed pods. I took care not to remove these as they are beneificial native species. As legumes, they share similar leaf characteristics as the invasive Silk Tree.

Source: https://iowaenvironmentalfocus.org/tag/invasive-species/

Boating - ensure that when you trailer your boat from a day out, you remove invasives from the boat trailer. Duckweed (Lemna sp.) commonly spreads this way as well as hydrilla (Hydrilla verticillata).

Iowa Invasive Plants: https://www.iowadnr.gov/conservation/forestry/forest-health/invasive-plants

Iowa Invasive Aquatics: https://www.iowadnr.gov/Fishing/About-Fishing-in-Iowa/Fighting-Invasive-Species

Invasive Inverts: https://www.iowadnr.gov/Fishing/About-Fishing-in-Iowa/Fighting-Invasive-Species/Aquatic-Invasive-Invertebrates

Invasive Fish: https://www.iowadnr.gov/Fishing/About-Fishing-in-Iowa/Fighting-Invasive-Species/Invasive-Fish

Invasive species good general background info: https://naturalresources.extension.iastate.edu/invasives